1. Field of the Invention
This invention relates to throttle valves for internal combustion engines, and more particularly to an improved process for producing such throttle valves.
2. Description of the Prior Art
Today in the automotive field, throttle valve units are increasingly being produced in large batches as injection-molded plastic components. For example, throttle valve units of this kind are valve housings manufactured by means of the injection molding process, with valve flaps that are injection molded into the housing. The throttle valve units that are used in the automotive field are subjected to temperatures between −40° C. and 120° C. so that care must be taken to assure the operational reliability of molded parts in this temperature range, specifically with regard to gap widths that can be achieved in the injection molding process.
EP 0 482 272 A1 discloses a valve device and a method for manufacturing a moving valve flap in a housing that accommodates the moving valve flap. The valve flap and valve housing are manufactured in the same mold. The housing is manufactured in a first injection molding step and the disk-shaped valve flap is molded inside it in a subsequent injection molding step. On the valve flap that moves in relation to the housing, sealing sections are provided, which cooperate in a sealing fashion with housing regions of the valve housing. The valve flap is preferably of the butterfly type and the valve housing is preferably of the type designed to accommodate a butterfly type valve flap. The disclosed manufacturing method is capable of significantly reducing the cost of producing a valve device for the automotive field. In this embodiment variant, the valve flap and its housing are positioned transversely in relation to the air flow direction.
U.S. Pat. No. 5,304,336 likewise relates to a method for manufacturing a valve device containing a moving part and a housing for accommodating the moving part. The moving part and the housing are produced in sequential manufacturing steps of an injection molding process. Preferably, the housing is injection molded in a first process step and the part that moves in relation to the housing is produced in another manufacturing step in which this moving part is situated in an at least partially closed position. According to the disclosed manufacturing method, a surface of the housing serves as at least a portion of a mold for forming a sealing portion of the movable valve flap, thus achieving a very close tolerance between the housing and the valve flap that moves in relation to it. Also according to U.S. Pat. No. 5,304,336, the valve flap that moves in relation to the housing is embodied as butterfly-shaped. The housing is preferably of the type that accommodates a butterfly type valve flap.
The manufacturing methods known from EP 0 482 272 A1 and U.S. Pat. No. 5,304,336 for producing an air-guiding part by means of an injection molding process have the disadvantage that these methods may produce molded parts that can be insufficient in their operational reliability. This is essentially due to an insufficient adjustability and reproduction precision of required gap widths in the shaft bearings and in the gas passage bore in devices manufactured in this way. With the methods described above, it is not sufficiently possible to selectively influence the gap width by means of machine setting parameters during formation, i.e. during the injection molding process, in order to achieve a definite air quantity in the closed position of the valve flap. From one production cycle to the next, the required gap widths cannot be sufficiently reproduced to attain a definite leakage air quantity in the closed position of the valve flap.
It is only permissible for the precision or uniformity of such gaps in valves to vary within a range of a few μm. This is of considerable importance in the automotive field in which such air-guiding parts are subjected to a larger range of temperatures from −40° C. to 120° C. (engine operation temperature in the cylinder head region). Due to a close interconnection between the temperature of the molding tool and the cycle time of the injection molding process according to the above-cited manufacturing methods, the required degree of precision cannot be achieved by means of the cavity provided in the molding tool. This is particularly true when, according to the methods in the embodiments described above, partially crystalline or amorphous thermoplastic high-temperature plastics are used for the above-indicated temperature range for applications in the engine compartment. According to the manufacturing methods known from EP 0 482 272 A1 and U.S. Pat. No. 5,304,336, it is not possible to react flexibly enough to process fluctuations, e.g. property fluctuations in the molding compounds during molding, i.e. the production process that is part of the injection molding process. The fluctuations described have an impermissibly powerful impact on the quality of the devices obtained.